Recently, along with the explosive increase of data and information handled by enterprises and government offices, the amount of data processed in large-scale computers such as host computers or storage subsystems coupled to servers and host computers or the capacity of the stored data is increasing rapidly. Especially, unlike servers and other information processing systems, storage subsystems are specialized for storing data, using a large number of hard disks as data storage media with a large capacity and high reliability, and which is managed and operated via a RAID (Redundant Array of Inexpensive Disks) system.
Further, the prices of storage subsystems are being reduced recently, along with which the hard disk drives (HDDs) used in the devices are switched from expensive and highly reliable fiber channel (FC) type devices to inexpensive SAS (Serial Attached SCSI) type devices and further to even more inexpensive SATA (Serial AT Attachment) type devices.
Further, the capacity of the HDDs are increased by the recent development of high density storage techniques. Along therewith, however, there are concerns that the frequency of occurrence of unrecoverable errors and recoverable errors may increase due to physical defects such as scratches formed on a media as the storage unit of the HDD (defects caused during fabrication or chipping caused by head contact) or read errors caused by insufficient recording, and the risk of data loss is increased.
Patent literature 1 discloses a method for saving data in HDD units when failure has occurred to an HDD as a technique for reducing the risk of data loss due to unrecoverable errors and the like. This technique monitors the failure occurring in one of the data disks of a plurality of data disks constituting one or more RAID groups, and when it is detected that a failure has occurred in a data disk, a correction copy is performed in a spare disk belonging to another spare disk group of the same RAID group as the data disk in which failure has occurred.